Barium titanate powders are widely used as dielectric materials for electronic elements such as PTC thermistors and piezoelectric devices, in particular, are suitable for dielectric layer of multi-layer ceramic capacitors (MLCCs). MLCCs are generally manufactured by alternately laminating a ceramic dielectric layer and an internal electrode layer, firmly pressing them, and then sintering to integrate them. Recently, electronic devices have become much smaller in size and higher in performance than ever before. For such specifications, the ceramic dielectric layer must be thin enough and the capacity of the capacitor must be sufficiently ensured etc. To meet these requirements, the following characteristics are required in the barium titanate powder.                (1) The powder should consist of ultrafine particles, and the particle size should be preferably in the range of 0.05 to less than 0.5 μm, more preferably in the range of 0.05 to less than 0.3 μm, and the particle size distribution should be narrow.        (2) The powder should have good crystallinity, specifically a perovskite structure, having a primary particle size ranging from 0.05 to 0.3 μm.        (3) The barium/titanium molar ratio should be near to 1.00, and particularly in the range of 0.99 to 1.01.        (4) The powder should have excellent dispersion characteristics when it is mixed into slurries or made into a paste.        (5) The powder should have good sintering characteristics.        
Barium titanate powders are usually produced by mixing titanium compounds and barium compounds, and then calcining the mixture at a temperature of not less than 1000° C. to carry out a solid phase reaction. However, in such a production process, the above chemical compounds are made to react at higher temperatures, and the particles of the obtained barium titanate powders become relatively large (for example, about 0.5 μm is the lower limit), the particle size distribution thereof is broad, and the shapes thereof are not uniform. Therefore, the conventional powders usually have to be sintered at a temperature of 1350° C. or even higher to produce dielectric ceramics having sufficient dielectric properties suitable for practical use. Hence, at such a sintering temperature, an expensive noble metal is needed to be used as the internal electrode material. In addition, the dispersion characteristics of the barium titanate powders in slurries are not so good. Such a broad range of particle size limits the thickness of the ceramic dielectric layer. As a production process, which can solve this problem, the liquid phase reaction method is known.
Various proposals have been made for this liquid phase reaction method. For example, U.S. Pat. No. 5,783,165 disclosed a production process in which barium titanate can be produced by a method comprising the steps of providing a solution comprising oxalic acid and titanium oxychloride, maintaining the solution at a predetermined temperature, adding barium carbonate to the solution thereby precipitating barium titanyl oxalate, and calcining the barium titanyl oxalate so as to prepare the barium titanate powder.
Another method for the production of barium titanate was presented in U.S. Pat. No. 4,670,243. This method produced very fine, very uniform barium titanate powders with extremely tight stoichiometric control and such powders can give good dielectric properties and need no complicated post-treatment on the product powders in order to adjust the stoichiometry. This process does, however, require the use of expensive tetra-isopropyl titanate as a precursor. As well as being somewhat expensive to manufacture, tetra-isopropyl titanate is a raw material difficult to work with due to its propensity to absorb atmospheric moisture. Further, it is difficult to completely remove the alkaline salts produced during the precipitation process due to the inherent problems associated with filtering extremely fine powders.
U.S. Pat. No. 6,129,903 and U.S. Pat. No. 6,409,983 disclosed some similar processes for producing barium titanate powders under hydrothermal conditions. The salts (e.g. carbonates), hydroxides of barium and titanium are combined in an aqueous mixture. The mixture is adjusted to a basic pH by adding alkaline metal hydroxides or ammonium hydroxide. This mixture is then reacted under hydrothermal conditions to produce crystalline barium titanate powders. The product slurry is cooled, filtered, and washed with water to remove impurities remaining from the salts and the pH adjusting chemicals.
U.S. Pat. No. 6,264,912 disclosed an improved hydrothermal production process in which barium titanate powders could be produced by a two-stage process. The first stage produces high quality monodispersed hydrous titania microsphere particles prepared by homogeneous precipitation via dielectric tuning in alcohol-water mixed solutions of inorganic salts. Titanium tetrachloride is used as an inorganic salt precursor material. The second stage converts the pure hydrous titania microsphere particles into crystalline barium titanate microsphere powders via low-temperature, hydrothermal reactions.
The above-mentioned processes have some disadvantages in that they involve several steps, require calcination or a long reaction time under high temperatures and high pressures to obtain crystalline powders, and require expensive precursors.
A primary object of the present invention, therefore, is to provide an improved process of preparing nanosized crystalline, spherical barium titanate particles having a narrow particle size distribution in one step.
Another object of the present invention is to provide a continuous production process operating in a water-based system and at low temperatures and normal pressures, having a high productivity and at the same time reducing operation costs and units or steps, giving high yields and a high conversion from the starting material to the end product.
A further object of the present invention is to provide an improved process of producing barium titanate powders having superior dielectric properties; low sintering temperatures; excellent dispersibility during the tape casting; and excellent uniformity of the grain sizes in the sintered product.
Still another object of the present invention is to provide barium titanate of tetragonal crystals having an average particle size of at most 200 nm by calcining the barium titanate powders obtained according to the above processes at an appropriate calcining temperature.